A person's perception of the environment responsive to sensory signals received from a sense organs such as an eye, an ear, or the skin, appears to be a subjective impression that the person's brain constructs to organize and correlate the sensory signals and provide the person with a conscious model that operates to interface the person with the environment. The interface enables the person to observe, record, and direct, his or her responses to features of the environment that are represented by the sensory signals.
The way the brain uses sensory signals from a given sense organ to generate a perception, and an efficiency with which it does so, are at least partially functions of learning, and different parts of the brain exhibit substantial plasticity in learning to generate perceptions responsive to signals from different sensory organs. For example, the occipital cortex of the brain in healthy people is responsible for providing visual perceptions and spatial models of an environment responsive to light collected from the environment by the eyes. In blind people who have suffered or been born with damage to the eyes or the neural system that transports signals from the eyes to the occipital cortex, the occipital cortex does not receive signals from the eyes. However, functional magnetic resonance imaging (fMRI) shows that the occipital cortex in blind people is often adapted to process audio signals generated by the ears and tactile signals generated by the fingers. The occipital cortex, by way of example, is generally involved in processing tactile signals produced by the fingers of blind people when they touch Braille letters to provide perceptions of the letters and read.
Blind people also appear to use the occipital cortex to generate spatial models of environments responsive to tactile or audio signals that are reminiscent of spatial models supported by visual perceptions in people who have normal sight. The spatial models that the blind appear to generate responsive to non retinal signaling enable them to function and navigate their environments in a manner that implies that their models share traits that characterize the models of sighted people.
The plasticity of the brain in learning to process sensory signals has been demonstrated in the development and use of SSDs that are designed to provide blind people with audio or tactile sensory input that substitutes for retinal signaling that they do not have. For example, in an SSD technology referred to as “vOICe”, images of an environment acquired by a video camera mounted in a pair of glasses worn by a blind person are encoded in auditory signals. The auditory signals are provided to the person by stereo speakers mounted in the glasses to aid the person in interacting with, and navigating in, the environment. Blind users of the glasses have reported and shown that the audio signals they receive from the SSD enable them to distinguish visual features, such as objects and patterns, of the environment.
Some tactile SSD technologies convert images from a glasses mounted camera to electrical signals on small electrodes arrayed in a tongue display unit (TDU) worn on the tongue. The electrical signals generated responsive to an image acquired by the camera stimulate tactile sensations on small regions of the tongue to generate an image, a “tongue image” on the tongue that represents the camera image. BrainPort® technologies of WICO Inc in Wisconsin USA reports developing a 3 cm×3 cm TDU having about 600 electrodes for generating tongue images. Blind people using the device appear to perceive the tongue images as low resolution images of their environment and are able to use the tongue images to distinguish such features as another person's fingers or to play tic-tac-toe on a large, (about 30 cm×30 cm) tic-tac-toe grid.